Interaction between an Adamantane End-Capped Poly(ethylene oxide

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Interaction between an Adamantane End-Capped Poly(ethylene oxide) and a β-Cyclodextrin Polymer Agnes Sandier, Wyn Brown,* and Holger Mays Department of Physical Chemistry, University of Uppsala, Box 532, 751 21 Uppsala, Sweden

Catherine Amiel Laboratoire de Recherche sur les Polyme` res, U.M.R. C7581, CNRS, 2-8 rue Henri Dunant, 94320 Thiais, France Received July 5, 1999. In Final Form: October 7, 1999 A novel route to the formation of large aggregates, with associated rheological enhancement, is provided by formation of inclusion complexes between a telechelic polymer and a second polymer containing appropriate receptor groups. The main focus of this paper describes such complexes between an adamantane end-capped poly(ethylene oxide) (PEO) (Mw ) 104 gmol-1) with a polymer of β-cyclodextrin (Mw ) 3.5 × 104 gmol-1) as studied by light scattering. There is a pronounced broadening of the width of the particle size distribution with increasing concentration of end-capped polymer, accompanied by a strong increase in the average relaxation time. Viscosity enhancement in the system was measured on the same samples. Newtonian behavior was observed in the shear rate range 0.017-90 s-1. Light scattering experiments (static and dynamic) were also made on the telechelic PEO itself. Light scattering shows the presence of a slowly relaxing component which dominates the scattering and this reflects large structures (radius 80 nm) created by interchain association to form a loose network, albeit at low concentration. Static and time-resolved fluorescence experiments show that there is no detectable tendency for “micellization” of the adamantane groups.

Introduction Aggregation of associative materials, such as the socalled hydrophobically modified polymers, is a phenomenon of widespread current interest and great utility. Recent studies encompass those materials made on the high molecular weight (but polydisperse) ethyleneoxideurethane copolymers (so-called HEUR materials),1-4 as well as those on hydrophobically end-capped polymers both in the absence5a,6 and presence5b of surfactants. The results from the latter well-defined systems (for instance, those described by Alami et al.7) are more easily interpretable than those from HEUR systems. Typical results have been interpreted in light of the variety of possible states of chain association ranging from “flower type” aggregates at low concentration to highly interconnected aggregates above the overlap concentration whose forma* Author to whom correspondence should be addressed. (1) Polymers in Aqueous Media; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (2) Polymers as Rheology Modifiers; Glass, J. E. Ed.; Advances in Chemistry Series 462; American Chemical Society: Washington, DC, 1991. (3) Water-Soluble Polymers for Petroleum Recovery; Back, J., Valint, P. L. Jr., Pace, S. J., Siano, D. B., Schulz, D. N., Turner, S. R., Eds.; Plenum Press: New York, 1986; Vol. 147. (4) Polymeric Materials Science and Engineering Proceedings; Nae, H. N., Reichert, W. W., Eds.; ACS Division of Polymeric Materials, American Chemical Society: Washington, DC, 1989; Vol. 61. (5) (a) Alami, E.; Almgren, M.; Brown, W.; Francois, J. Macromolecules 1996, 29, 2229. (b) Alami, E.; Almgren, M.; Brown, W. Macromolecules 1996, 29, 5026. (6) Chassenieux, C.; Nicolai, T.; Durand, D. Macromolecules 1997, 30, 4952. (7) (a) Alami, E.; Rawiso, M.; Isel, F.; Beinert, W.; Binana-Limbele, W.; Francois, J. In Hydrophilic Polymers; Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996; Chapter 18, p 343. (b) Francois, J. In Progress in Organic Coatings; Elsevier: New York, 1994, 24, 67. (c) Benkhira, A.; Franta, E.; Rawiso, M.; Francois, J. Macromolecules 1994, 27, 3963.

tion is associated with a dramatic increase in the solution viscosity. Relevant studies are also those on short poly(ethylene oxide) (PEO) chains linked to a poly(styrene) block,8 PEO chains terminated in pyrene units,9 and network formation in water-in-oil microemulsions stabilized by a triblock copolymer.10 The present paper focuses on a different type of hydrophobically driven interaction involving a highly specific matching between groups on the component polymers, namely the formation of multiply linked inclusion complexes. The unique feature here is the nature of the link formed between a β-cyclodextrin (β-CD) polymer11 and an adamantane-modified PEO. The adamantane group was selected because it fits precisely into the slightly apolar β-CD cavity,12 consisting of a ring structure of seven glucopyranose units, with a well-defined internal diameter of 7.8 Å (see Figure 1b). In aqueous solution, the cavity is occupied by water molecules which are energetically unstable and are readily substituted by a guest molecule with appropriate dimensions which is less polar than the water molecules. It is expected that large structures will result on formation of such links, leading to important changes in the character of the suspensions, e.g., viscosity enhancement. The polymers used here are both welldefined in molecular weight distribution which facilitates light scattering data interpretation. An earlier study was devoted to the interactions between a polydisperse ada(8) Mortensen, K.; Brown, W.; Almdal, K.; Alami, E.; Jada, A. Langmuir 1997, 13, 3635. (9) Burrows, H.; Brown, W.; Almgren, M. to be submitted for publication. (10) Mays, H.; Almgren, M.; Brown, W. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1648. (11) Amiel, C.; Se´bille, B. Adv. Colloid Interface Sci. 1999, 79, 105. (12) Eftink, M. R.; Andy, M. L.; Bystrom, K.; Perlmutter, H. D.; Kristol, D. S. J. Am. Chem. Soc. 1989, 111, 6765.

10.1021/la990873a CCC: $19.00 © 2000 American Chemical Society Published on Web 01/05/2000

Adamantane End-Capped PEO and a β-Cyclodextrin

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with an average aggregation number of the terminal groups of about 15-30. Experimental Section

b

c

Figure 1. Schematic structures of (a) PEO-AD, (b) the β-CD molecule, and (c) the complex formed between a short-chain β-cyclodextrin polymer and PEO-AD.

mantane-substituted copolyester and a β-CD polymer of high molecular weight.13 Because, however, the telechelic polymer itself is expected to self-associate, this paper also describes light scattering measurements made to elucidate this aspect of the adamantane end-capped polymer. These results are discussed prior to the results on its mixtures with the β-CD polymer. It was found that there are pronounced differences between the association behavior of the present material and the analogous PEO chains with terminal dodecyl moieties previously examined.5 In the latter system, micelle-like aggregates of the end-groups exist (13) Moine, L.; Amiel, C.; Guerin, P.; Brown, W. Langmuir, submitted for publication.

1. Materials. PEO (Mw ) 104gmol-1) was purchased from Merck (Nogent-sur Marne, France). The hydrophobically modified PEO was obtained by reacting the terminal -OH groups with 2.5 excess 1-adamantyl isocyanate (Aldrich, St Quentin Fallavier, France). PEO was previously dried by heating under vacuum at 70 °C overnight. The reactants were dissolved in dichloroethane and two catalysts were added: dibutyltin dilaurate (Merck) and triethylamine (Aldrich). The reaction mixture was heated at 65 °C for 6 h. After removal of the solvent, the reaction product was dissolved in distilled water. The aqueous solution was purified by addition of activated carbon and successive filtrations, and then freeze-dried. The resulting polymer (abbreviated PEO-AD) was recovered with a 70% yield. The degree of substitution was found to be 90% through 1H NMR measurements. The β-CD was prepared by polycondensation with epichlorohydrin under strong alkaline conditions. Synthesis and characterization were described previously.14 The polymer used for this study had a weight average molecular weight 3.5 × 104 gmol-1 as determined using size exclusion chromatography in equivalent pullulan. 2. Methods. Static Light Scattering Measurements. A Hamamatsu photon-counting device with a 3mW He-Ne laser was used. Toluene was used as the reference scatterer (Rref ) 13.59 × 10-8 m-1 at λ ) 632 nm); K ) 4π2n2(dn/dc)2/NAλ4, where the refractive index increment (dn/dc) was measured in a differential refractometer with Rayleigh optics. NA is Avogadro,s constant and λ is the wavelength. For the present systems PEO and adamantane end-capped PEO in water, dn/dc was 0.131 mLg-1 at the wavelength used and 25 °C. R90 is the Rayleigh ratio at angle 90° determined using [(I-Io)/Iref]Rref‚(n/nref)2. Here n ) 1.33 is the solvent refractive index and nref is that of toluene. I is the measured total time-averaged scattered intensity, Io is that of the solvent, water, and Iref is that of toluene. The dynamic light scattering setup consists, as described previously,15 of a 488 nm Ar ion laser and detector optics which include an ITT FW 130 photomultiplier and ALV-PM-PD amplifier-discriminator connected to an ALV-5000 autocorrelator built into a computer. Solutions of each polymer were prepared by dissolving the appropriate amount of polymer in purified water (Millipore SuperQ-System). Concentrations are expressed throughout as C% (w/ w). Solutions were either filtered through Sartorius Minisart filters (cellulose acetate membranes) of 0.2 µm pore diameter or 0.1 µm Anotop filters (aluminum oxide) in the case of the pure components. The cylindrical scattering cells were sealed and then immersed in a large-diameter thermostated bath containing Decalin. Measurements were made at different angles in the range 30-140°, and at different sample concentrations. Data analyses were performed by fitting the experimentally measured g2(t), the normalized intensity autocorrelation function, which is related to the electrical field correlation function g1(t) by the Siegert relationship:

g2(t) - 1 ) β|g1(t)|2

(1)

where β is a factor accounting for deviation from ideal correlation. For polydisperse samples, g1(t) can be written as the inverse Laplace transform (ILT) of the relaxation time distribution, τA(τ) (see ref 15):

g1(t) )

∫

∞

-∞

τA(τ)exp(-t/τ)d ln τ

(2)

where t is the lag-time. The relaxation time distribution, τA(τ), is obtained by performing ILT using the constrained regularization algorithm REPES,15,16 which minimizes the sum of the (14) Renard, E.; Deratani, A.; Volet, G.; Se´bille, B. Eur. Polym. J. 1997, 33, 49. (15) Schille´n, K.; Brown, W.; Johnsen, R. M. Macromolecules 1994, 27, 4825. (16) Jakes, J. Czech J. Phys. 1988, B38, 1305.

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squared differences between the experimental and calculated g2(t). A mean diffusion coefficient D is calculated from the second moment of each peak as D ) Γ/q2, where q is the magnitude of the scattering vector q ) (4πn/λ)‚sin(θ/2) and Γ ) 1/τ is the relaxation rate of each mode. Here θ is the scattering angle, n is the refractive index of the medium, and λ is the wavelength of the light in a vacuum. Within the dilute regime, D varies linearly with the concentration, C, that is,

D ) D0(1 + kDC + ...)

(3a)

where D0 is the diffusion coefficient at infinite dilution and kD is the hydrodynamic “virial” coefficient related to the second virial coefficient A2 by

kD ) 2A2M - kf - 2v2

(3b)

Here M is the molar mass, kf defines the concentration dependence of the friction coefficient in f ) f0(1+kfC+...), and v2 is the partial specific volume. The Stokes-Einstein equation relates D0 to the hydrodynamic radius (Rh):

Rh ) kT/(6πηD0)

(4)

where kT is the thermal energy factor and η is the temperaturedependent viscosity of the solvent. Molecular weights and radii of gyration were also estimated for the individual components in the relaxation time distribution. The procedure was as described above for static light scattering, but using here Rref ) 4.0 × 10-7 m-1 at λ ) 488 nm. The intensities were obtained by apportioning the total intensity according to the relative amplitudes obtained in the Laplace inversion routine. Viscosimetry. Measurements were done with a low-shear Couette flow type rheometer, Contraves LS 30, working at shear rates of 0.017-90 s-1. Fluorescence Measurements. a. Steady-State Fluorescence. Steady-state fluorescence measurements were acquired with a SPEX Fluorolog 1680 instrument combined with a SPEX Spectroscopy Laboratory Coordinator DM1B. The fluorescence spectra were measured between 350 and 650 nm at an excitation wavelength of 320 nm. b. Luminescence Quenching. The time-resolved luminescence decays were obtained with a previously described single-photon counting setup.10 Pyrene and 3,4-dimethylbenzophenone (DMBP) were used, respectively, as the fluorescence probe and quencher.

Results and Discussion Comparison of PEO and PEO-AD Polymers. a. Fluorescence Measurements. Aqueous solutions of PEO-AD were investigated at the polymer concentration of 5% using static and time-resolved fluorescence measurements. Pyrene was chosen as a suitable probe due to its strong hydrophobic character and its sensitivity to the environment polarity. In preliminary attempts to dissolve pyrene in the polymer solution it was observed that, already at c ) 0.4 mol‚L-1, the solutions were turbid from dispersed pyrene crystals, clearly showing a very limited solubility for nonpolar molecules. This contrasts with micellar solutions in which hydrophobic solutes dissolve readily. The static fluorescence measurements with pyrene and PEO-AD did not show any difference from that of pyrene in pure water. The intensity ratio I(III)/I(I) of the third and first vibrational peaks in the pyrene emission spectrum remained unchanged, which contrasts with pyrene solubilized in micelles.17 Thus, the environment of pyrene molecules is not influenced by the presence of the endcapped polymers. This again indicates that discrete aggregates are not present in the solution. This finding (17) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

is further corroborated by the time-resolved measurements, where the introduction of the quencher DMBP 3,4dimethylbenzophenone did not influence the decay behavior. An additional quenching process from the confinement of probe molecules and quenchers within the same aggregate did not occur (as being typical for micelles), nor was probe lifetime decreased as compared to a polymerfree solution. It is obvious that pyrene is not confined in an hydrophopic environment as would be the case in the presence of discrete micelle-like aggregates. The present system thus differs strongly from the previously studied dodecyl end-capped PEO of the same molar mass.5 In that study it was established using fluorescence methods that the aggregation number of the hydrophobic domains was in the range of 15-30 end groups per micelle. Here, using the same technique, we conclude that hydrophobic domains do not exist in the telechelic polymer solutions and aggregates having the same character as observed in the dodecyl-PEO system do not occur. Because there is an interaction among the PEO-AD molecules, as we show below, the results can be rationalized by assuming a “loose” network of the associating polymer with a low density of connection points. These are important features because we sought to induce network formation in the system through interaction of the adamantane groups with matching receptor sites on β-CD polymers. b. Dynamic Light Scattering Measurements. Measurements were initially made both on the unmodified linear PEO (Mw ) 104 gmol-1) as well as on the same polymer modified with hydrophobic adamantane groups at each end (PEO-AD). Figures 2a,b compare, respectively, time correlation functions and ILT relaxation time distributions for unmodified and adamantane-end-capped PEO in aqueous solution at C ) 1 wt %. The distributions are typically bimodal for both unmodified and modified polymers. However, the size distribution for the PEO-AD has a much higher relative intensity contribution from a component with long relaxation time compared to PEO itself, in addition to the fast component corresponding to the single coil. In each case, the relaxation rates of both fast and slow modes were shown to be proportional to the square of the scattering vector from measurements in the angle range 30°-130°. From the diffusion coefficients at infinite dilution, the hydrodynamic radii were calculated using eq 4 and the resulting values are given in Table 1. The experimental Rh values for the fast modes (Table 1) are very close to the value calculated for the unmodified PEO chain of the same molecular weight obtained using the empirical Devanand-Selser relationship17 in water at 30 °C:

Rh ) 0.0145 Mw0.571(0.009 (nm)

(5)

This relationship gives Rh ) 2.79 nm for Mw ) 104 gmol-1. Thus, it is concluded that the fast mode corresponds to unassociated PEO chains and that the presence of the adamantane end groups has an insignificant effect on the single coil radius. Although similar experimental Rh values are found for the fast mode of both the precursor polymer and the telechelic polymer, the hydrodynamic radius will be insensitive to small conformational changes, for example, if compact flowerlike aggregates are formed. We find, however, good agreement between the determined molecular weight for the fast mode of the modified chains (see Table 2) and that for unmodified PEO, which demonstrates the absence of such highly compact aggregates.

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Figure 2. (a) Time correlation functions (g2(t)) from dynamic light scattering for the linear PEO (Mw ) 104 gmol-1) (squares) and the adamantane-modified material (inverted triangles) at concentration 1%. (b) Relaxation time distributions (ILT) for PEO (Mw ) 104 gmol-1) (open circles) and the PEO-AD (squares), corresponding to the correlation functions in Figure 2a. Table 1. Rh Values for Slow and Fast Modes of PEO and PEO-AD sample

Rh (nm) (fast mode)

Rh (nm) (slow mode)

PEO PEO-AD

3.0 3.0

80 80

Table 2. Characterization of PEO-AD (Mw and Rg of the Fast and Slow Modes) by a Combination of SLS and DLS Methods sample

Rg (nm) (slow mode)

Mw (SLS) (average)

Mw (DLS) (fast mode)

Mw (DLS) (slow mode)

PEO-AD

100

90000

9000

240

The relaxation time distributions for PEO typically have a low amplitude slow component (see Figure 2b) over the concentration range studied (0.01-10%). Clusters are a well-known feature of PEO aqueous solutions18 and it has been shown19 that such clusters are nucleated by impurities and can be removed using hydrophobic filters or very small pore diameter filters (0.02 µm). Nevertheless, there does not yet seem to be a consensus as to the origin of the impurities themselves. (18) (a) Devenand, K.; Selser, J. C. Macromolecules 1991, 24, 5943. (b) Devenand, K.; Selser, J. C. Nature 1990, 343, 739. (19) Porsch, B.; Sundelo¨f, L.-O. Macromolecules 1995, 28, 7165.

Figure 3. (a) Relaxation time distributions for different concentrations of PEO-AD. (b) Diffusion coefficients versus concentration for PEO-AD (angle θ ) 90 and 25 °C).

Figure 3a shows relaxation time distributions for different concentrations of end-capped PEO. The strictly bimodal nature of the distributions obtained using Laplace inversion was substantiated by directly fitting the autocorrelation functions to a double Kohlrausch-WilliamsWatts (KWW) function (of the form of eq 6; see below). The correlation functions are well fit with each component described by a stretched exponential function with a β value of approximately unity, giving further credence to the results from Laplace inversion. Use of high-precision Anotop (aluminum oxide) 0.02 µm pore diameter filters on solutions of the end-capped polymers did not, in this case, eliminate the slow mode even though the particles are substantially larger than the membrane pore size. Because this latter component is immediately observed

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Figure 4. Slow mode scattered intensities for linear PEO (Mw ) 104 gmol-1) (filled circles) and adamantane-modified material of the same chain length (open squares) as a function of polymer concentration.

again after filtration, the corresponding aggregates clearly reform after passing the filter. As seen in Figure 3b, the fast mode has the typical positive concentration dependence for a hydrophilic polymer in an aqueous medium (large value of A2 in eq 3b characterizing the thermodynamic interactions), whereas the slow mode has a negative concentration dependence owing to the dominant dependence of the frictional term for large particles. Figure 4 shows the concentration dependence of the slow mode intensity for the PEO-AD polymer (obtained by multiplying the measured time-averaged scattered intensity by the slow mode amplitude) and shows that the scattered intensity of the slow component of the PEO-AD is significantly greater than that for the precursor polymer at the same weight concentration. It should be borne in mind, however, that only a small weight fraction of the chains is represented in the aggregates of PEO-AD. A simple estimate of the ratio of the mass concentrations ascribable to the two modes, Cs/Cf, (where subscripts s and f refer to slow and fast modes) can be obtained by dividing the ratio of the peak amplitudes by the relative molar masses. The latter may be obtained from the ratio of the corresponding relaxation rates by making the coarse assumption that D ) Γ/q2 ∼ M-0.5. This yields Cs/Cf ∼ 0.5 × 10-2. Despite its very low weighting, some discussion of the apparently dominant slow relaxation in PEO-AD solutions is considered meaningful here, because we are concerned below with large particles formed through the interactions with the β-CD polymer. At present we are unable to explain why association yields such an apparently well-defined slow component (Figure 2b) because one might have anticipated a broad distribution of the aggregate size. Preliminary static and time-resolved fluorescence experiments performed on the PEO-AD system showed that there is no detectable tendency for micellization of the adamantane groups as was previously found5 in solutions of dodecyl-end-capped PEO. The angular dependences of the fast and slow components were also measured for the adamantane telechelic polymer, using a combination of static and dynamic light scattering. Several concentrations over the range 0.16% were used. From the light scattering time-averaged intensity data, an average value of the apparent molecular weight was obtained (Table 1). However, from the

Sandier et al.

simultaneous static and dynamic light scattering experiments, it was also possible to deduce the molecular weights corresponding to the individual modes by apportioning the total intensity between the different relaxing components. The value of Mw ) 9 × 103 for the fast mode agrees in magnitude with that expected for the single chain. The slow mode molecular weight (2.4 × 105) would correspond to about 27 chains per aggregate. Values of the radius of gyration, Rg’, for the slow mode were estimated from the ratio of slope to intercept of plots 1/(I sin θ) ) f[sin2(θ/2)] and are included in Table 2. Effect of Salt Addition or Organic Solvent. To gain a better understanding of the origin of the slow modes, the effects on the scattered intensities of added salt (KCl, 0.05 M) and the use of an organic solvent (dioxane) were examined. In both cases, the slow mode was not removed: a small increase of the hydrodynamic radius (140 nm) was correlated to a small increase of the associated intensity. This relative insensitivity to salt or dioxane shows that the aggregates corresponding to this mode do not result from hydrophobic interactions. This is in agreement with the fluorescence results. We conclude from the above results that the aggregates in water, salt solution, and dioxane, corresponding to the slow relaxation, may derive from strong hydrogen bonding involving the penultimate -CO-NH- moieties linking the PEO chain to the adamantane groups (see Figure 1), instead of reflecting interactions between adamantane groups. This conclusion is supported by the work of Alami et al.20 who observed that the urethane linkage influences the associative behavior of alkyl end-capped PEO in aqueous solution. The apparently very weak interactions between adamantane groups may be a consequence of their bulky nature leading to unfavorable steric conditions. β-CD polymer was prepared having an average molecular weight of 3.5 × 104 according to size exclusion chromatography (SEC) (pullulan equivalent). The β-CD content was determined by 1H NMR as described in ref 14 and was found to equal 41%. When epichlorohydrin is used to couple the cyclodextrin units, the cavities will become linked in a highly random conformation on forming the polymer because each β-CD monomer has 21 exterior hydroxyl groups (7 primary hydroxyl at one rim and 14 secondary hydroxyl at the other rim) with different possible orientations available for reaction. Under the conditions of the synthesis, the reaction occurs preferentially on the more accessible primary hydroxyl.14 Despite its hydrophilicity, the polymer thus has a highly compact conformation. Laplace inversion of the correlation functions revealed a bimodal relaxation time distribution, with the slow peak corresponding to a hydrodynamic radius of 90 nm. It was possible to remove the latter slowly relaxing component by filtration through a 0.1 µm pore diameter membrane. The absence of concentration dependence for the diffusion coefficient of the resulting well-defined component (data included in Figure 7a below) is expected for a compact highly coiled chain; the hydrodynamic radius is 6.0 nm. The molecular weight determined by light scattering, Mw ) 100 000, is three times higher than that determined by SEC (pullulan equivalent) before filtration, which supports the compact nature of this branched polymer. Mixtures of β-CD and PEO-AD Polymers. In mixtures of β-CD and PEO-AD polymers, one expects strong interaction between the two polymers because it was earlier established11,12 that the adamantane group fits (20) Zhang, H.; Hogen-Esh, T. E.; Boschet, F.; Margaillan, A. Langmuir 1998, 14, 4972.

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precisely into the β-CD cavity to yield an inclusion complex with the interaction driven primarily hydrophobically (see Figure 1). The latter potentially leads to the formation of aggregates made up of short β-CD polymer chains interlinked by adamantane-terminated PEO molecules. Marked changes in the relaxation time distributions of the mixtures are anticipated and should correlate with the viscosity behavior observed. a. Dynamic Light Scattering Study. Figure 5a summarizes the changes in the relaxation time distributions for different concentrations of PEO-AD in the 1% β-CD polymer solution. The distributions remain single-peaked but broaden significantly with PEO-AD concentration increase. This behavior is illustrated in Figure 5b by the decrease of the exponent β obtained on analyzing the autocorrelation functions with a single KWW stretched exponential function:

g2(t) ) A exp(-(t/τ)β)

(6)

in which β (0 < β < 1) is a fitting parameter which describes the breadth of the distribution and τ is the average relaxation time. The associated stretched exponential (β) decreases from a value close to unity typifying diffusion (see Figure 5b) for the pure β-CD polymer and the average relaxation time becomes progressively longer on addition of PEO-AD. When Figures 5a and 3a are compared, the trend observed in the relaxation time distributions could be interpreted as the result of a simple overlap of the β-CD polymer mode and the slow mode of PEO-AD. However, at low PEO-AD concentration (e0.5%) the intensity of the PEO-AD slow mode is negligible compared to that of the β-CD polymer mode, but an increase of the mean relaxation time is still observed. Thus, the single broad mode must be a result of the inclusion complex formed between the two polymers, as is supported by the viscosity measurements described in the next section. We note here that the hydrophobic interactions leading to the inclusion complex apparently suppress the aggregates of PEO-AD which are present in aqueous solution, i.e., the addition of β-CD polymer functions as a structurebreaker for the PEO-AD aggregates.20 Figure 6a shows time correlation functions for the highest PEO-AD concentration (1% β-CD polymer plus 2% PEO-AD) in the low angle range where the process is diffusive, i.e., the relaxation rate is linearly dependent on q2. Figure 6b illustrates the approximate q-independence of the relaxation rate at high angles (i.e., probing shorter length scales) in the range above 100°. This behavior suggests that two dynamic processes are accessible depending on the q-range used. Figure 6c shows the average relaxation time as a function of the squared scattering vector. At high q, the limiting relaxation time is about 230 ms. The concentration fluctuations thus relax by diffusion (q2-dependent relaxation rate) until the characteristic relaxation time approaches that for the inclusion link (qindependent relaxation rate). At the crossover, the characteristics of a mixed mode will be observed. Thereafter at higher q, the concentration fluctuations will become q-independent with the relaxation rate determined by the average lifetime of the links in the complex. Because many complexing links are involved in each particle, a broad distribution results. Figure 7a shows apparent diffusion coefficients for mixtures at different constant molar concentrations of adamantane groups as a function of β-CD polymer (expressed as the monomer concentration). The slopes are

Figure 5. (a) Relaxation time distributions for (bottom): low MW β-CD polymer at C ) 1% and, respectively, the complex formed with increasing concentration of PEO-AD. (b) KWW stretched exponential (β in eq 6) for the distributions shown in Figure 5a as a function of increasing concentration of PEO-AD in the mixtures.

positive at the low PEO-AD concentrations in contrast to the zero slope in the case of the pure β-CD polymer. In

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Figure 6. Time correlation functions for the mixture of the β-CD polymer at C ) 1% and PEO-AD 2% at different angles: (a) 40-80°, (b) 100-120°, and (c) average relaxation time (τ from eq 6) as a function of q2 showing its constancy in the high angle region.

Sandier et al.

Figure 7. (a) Diffusion coefficients of the inclusion complex as a function of β-CD concentration for the β-CD polymer (open circles) and mixtures with PEO-AD polymer as shown. Concentrations of the polymers are expressed as moles β-CD monomer and moles adamantane group. (b) Hydrodynamic radius for the complex aggregate (from the infinite dilution values of D for the β-CD polymer in Figure 8a) as a function of moles adamantane group present in the form of PEO-AD polymer. (c) Apparent hydrodynamic radius for the complex. Concentrations of the polymers are expressed as moles β-CD monomer and moles adamantane group.

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Figure 8. Mechanism of association: decorated β-CD polymer (visualized as a sphere) in the presence of excess PEO-AD. As more PEO-AD is added, the particle size increases due to a higher grafting density.

terms of eq 3b, the positive slopes reflect enhanced virial interactions, i.e., greater solute-solvent interactions accompany significant particle growth, as may be deduced from the different intercepts in Figure 7a. Rh determined at infinite dilution of β-CD polymer is shown as a function of moles adamantane group in Figure 7b. The apparent hydrodynamic radius of the complex increases strongly with the adamantane group concentration from an initial value of 6 nm to a value of about 30 nm. Justification for interpretation as a change in particle size follows from Figure 7a, which shows that the concentration dependences (and thus the virial interactions) remain similar on altering the adamantane concentration. At infinite dilution of β-CD polymer, the β-CD polymer chains do not interact with each other and the mean distance between them is large enough to avoid connections between PEO chains. Thus, the Rh values may be considered to correspond to isolated β-CD polymer molecules “decorated” with PEO-AD. Although Rh increases with the PEO-AD concentration, it must be borne in mind that Rh is the apparent dimension in the presence of PEOAD polymer. The complexation constant, determined in a previous work,11 is of the order 2000-3000 l‚mol-1. In the concentration range used, more than 80% of the cavities are bound to adamantyl moieties. Thus, the coverage of β-CD particles does not change with the PEOAD concentration. A possible explanation could be the mechanism shown in Figure 8. At low PEO-AD concentrations, both ends of a PEO-AD chain should be linked to the same β-CD chain. As the concentration increases, more PEO-AD chains are involved in complexation and this favors linking by only one PEO-AD end, which could be a contributory factor leading to an increase of the true particle size. Analogous experiments were made by keeping the concentration of β-CD polymer constant and varying the adamantane concentration, as shown in Figure 7c: The apparent radius of the particles increases as a function of the concentration of each polymer. Extrapolation to infinite dilution in this case, gives an Rh value which corresponds to an isolated PEO-AD chain linked to β-CD polymers at its ends. An Rh value of about 9 nm (Rh (CD-polymer) + Rh (PEO-AD)) agrees with this assumption. Measurements were also made on the mixture with highest PEO-AD concentration (1% β-CD polymer with 2% PEO-AD) in the presence of an excess of monomeric β-CD which should serve to cap exposed adamantane groups and effectively inhibit interaction with the cyclodextrin polymer cavities. The relaxation time reverted to the position shown by the vertical dotted line in Figure 5a (bottom), showing that the interactions causing complex formation are then insignificant.

Figure 9. Specific viscosity of a mixture containing 1% β-CD polymer with 2% PEO-AD as a function of shear rate at 20 °C (2), 25 °C (O), 30 °C (B), 40 °C ([), 45 °C (0).

Measurements were also made on the same system at temperatures of 35 and 45 °C. It was found that the apparent hydrodynamic radius decreased weakly with increasing temperature, which is in line with the observed decrease in the relative viscosity. b. Viscosity Study. Association between the two components is also shown by viscosity measurements. At 25 °C, a solution containing a mixture of 1% β-CD polymer and 2% PEO-AD, prepared in the same way as for the light scattering experiments, has a zero shear specific viscosity of 28.5. This value is 50 times higher than the specific viscosity of a 2% PEO-AD solution and 285 times higher than the specific viscosity of a 1% β-CD polymer solution. Newtonian behavior was observed in the shear rate range 0.017-90 s-1 and at all temperatures used (20-45 °C), as is shown in Figure 9. This behavior differs strongly from that of alkyl-terminated PEO where shear thinning properties are clearly observed over the same shear rate range. The latter is expected when a reversible network is formed due to interchain association.5,7 The insensitivity to shear rate in the present system tends to prove that, instead of a reversible network, aggregates of a limited size are formed (see, for instance, Figure 8, where “decorated” CD polymers are considered as the limiting case). The viscosity of the mixture decreases as a function of temperature (Figure 10), following an Arrhenius relationship:

η ) A expEa/kT η0

(7)

η0 is the solvent viscosity at temperature T, and Ea is the activation energy. A value of Ea ) - 41 kJ/mol described the best fit to this relationship, which is in good agreement with the enthalpies of inclusion complexes of adamantane compounds with β-CD described in the literature (20-40 kJ/mol) (see ref 12). Conclusions Adamantane end-capped short chain (Mw ) 104 gmol-1) PEO in solution self-associates to form a low number density of large, loosely structured particles, the interior of which are probably network-like, in contrast to the behavior of the analogous molecule with dodecyl end

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Figure 10. Specific viscosities as a function of temperature for a mixture containing 1% β-CD polymer with 2% PEO-AD (0), for 2% PEO-AD solution (2), and for 1% β-CD polymer solution ([).

groups, where discrete micelle-like regions are formed through the association of terminal hydrophobic moieties. The results suggest that the association may derive from strong hydrogen bonding involving the penultimate -CONH- moieties linking the PEO chain to the adamantane groups, rather than reflecting interactions between adamantane groups. Mixtures of PEO-AD with the β-CD polymer yields complexes where the interpolymer links are formed by specific inclusion of the adamantane groups in the β-CD cavities. Interaction between the two polymers has been

Sandier et al.

demonstrated through dynamic light scattering and viscosity measurements. The increase in average relaxation time of the aggregate, and the corresponding hydrodynamic radius at infinite dilution, correlate with the viscosity enhancement. Two limiting cases have been examined: a β-CD polymer decorated with PEO-AD chains when PEO-AD is in large excess, on one hand, and single PEO-AD chains linked to β-CD polymers at its ends in the case where the β-CD polymer is in great excess, on the other. The angle dependences of the relaxation rates of the component mixtures show a crossover between diffusive behavior and q-independent behavior at high q, where the corresponding relaxation time reaches a limiting value of about 230 ms. This value may be related to the lifetime of an inclusion link. With increasing temperature, the viscosity decreases substantially in agreement with the light scattering results, due to a weakening of the interaction strength. The mixtures exhibit Newtonian behavior at shear rates lower than 100 s-1, as opposed to the situation usually observed with associative thickeners. These effects can be attributed to the formation of aggregates of limiting size rather than a reversible network. Acknowledgment. Bo Rydins Stiftelse fo¨r Vetenskaplig Forskning is thanked for support of this work and financing the stay of Agnes Sandier at the Department of Physical Chemistry in the University of Uppsala. LA990873A